Fluid seals between a rotating component and a static component are used in gas turbine engines to restrict the flow of a fluid from a region of high pressure to a region of lower pressure. As shown in FIG. 1, an air-riding seal system 1 typically comprises a first sealing ring 2 mounted on a rotor 3 which faces a second, static sealing ring 4 which is mounted in a housing 5 and biased towards the first sealing ring 2 by an axial spring 9.
When the rotor 3 is stationary, the sealing face 6 of the first sealing ring 2 is pressed against the sealing face 7 of the second sealing ring 4 thus preventing leakage from the high pressure region to the low pressure region.
As shown in FIG. 2, the sealing face 6 of the first sealing ring 2 has a series of spiral grooves 18. As the first sealing ring 2 rotates with the rotor 3, the spiral grooves 18 create a lift force which results in an air film between the sealing face 6 of the first sealing ring 2 and the sealing face 7 of the second sealing ring 4. This air film maintains the seal between the high pressure and low pressure regions whilst substantially eliminating wear on the sealing surfaces 6, 7 of the sealing rings 2, 4.
Due to manufacturing tolerances, build tolerances, bearing clearances and engine-running deflections, there is a tendency for the mounting face 8 of the first sealing ring 2 (and therefore the sealing face 6) to be angled away from an orientation perpendicular to the axis of rotation 10 of the rotor 3 as shown in FIG. 3. This results in the first sealing ring 2 acting like a swash-plate with the distance between the sealing surface 6 on the first sealing ring 2 and the sealing surface 7 on the second sealing ring 4 having positional and time variations which can result in failure of the air film seal and/or contact between the two sealing rings 2, 4. To avoid these variations and seal failure/sealing ring contact, it is necessary for the second sealing ring 4 to track the first sealing ring 2. This can lead to wear or even failure of the secondary seal.
Additional sealing (e.g. an elastomeric seal) is provided between the second sealing ring 4 and housing 5 to ensure sealing functionality of the system is maintained.
Furthermore, any deformities in the material of the rotor 3 (e.g. arising from circumferential weave), may be transmitted through the mounting face 8 of the first sealing ring 2 through to the sealing surface 6 of the first sealing ring 2 causing variations in the spacing between the sealing surfaces 6, 7 which reduces the efficiency of the air-film seal.
Accordingly, there is a need for an air-riding seal arrangement that allows a reliable seal between a rotating component and a static component that reduces wear on seal components and reduces the effects of any deformations in the rotor material on sealing efficiency.